Some heat for the marines (5)
I believe the answer is:
therm
'some heat' is the definition.
(Therm is a unit of heat energy)
'the marines' is the wordplay.
'marines' becomes 'RM' (abbreviation for 'Royal Marines').
'the'+'rm'='THERM'
'for' is the link.
(Other definitions for therm that I've seen before include "Unit of heat and gas measure" , "Unit of measurement of heat" , "Unit of heat as in gas supply" , "Unit of heat energy" , "Gasman's unit" .)